This invention relates to inorganic resist materials. More particularly, this invention relates to an inorganic resist material which exhibits a differential etch rate when exposed to a heat source.
A resist is a material which changes its solubility in a particular solvent after exposure to a source of energy such as heat, light or electron beams for example. A layer of a resist is applied to a substrate, exposed partially to an energy source and developed by immersion in an etchant. The difference in etch rates for the exposed versus unexposed portions of the film results in partial dissolution of the resist film to expose portions of the substrate beneath. The substrate not covered by the resist can then be etched away, and the resist removed, resulting in a surface relief pattern in the substrate.
Generally, resist materials are used in the electronics industry to define surface relief patterns in semiconductor materials. With the increasing complexity of integrated circuits and integrated circuit technology, there is a great need for a resist material which does not swell upon development and thus limit the resolution and definition of lines or circuit elements. Development refers to the process of removing the exposed portion of a positive resist material or the unexposed portion of a negative resist after exposure to light. The chemicals used for the removal of the resist tend to swell the remaining material.
Photoresist materials are generally sensitive to light of specific wavelengths and require a particular light source which emits light at the wavelength sensitivity of the resist. Electron beam resists, although providing superior sensitivity than most light sensitive resists, are complex and require more expensive and complicated machinery to be used effectively.
Polycrystalline silicon, silicon nitride, and silicon dioxide have been used as masking layers in the processing of silicon on sapphire devices. Patterns in said materials have been formed by standard photolithographic techniques, for example, see U.S. Pat. Nos. 3,698,947 and 3,721,584. Recently, Laff et al in IEEE Transactions On Electron Devices, Vol. ED-21, No. 11, November, 1974, p. 743, reported that heating a CVD deposited silicon film with a Gaussian-focussed continuous wave argon-ion laser to about 1000.degree. C. at a scan of about 10 centimeters per second rendered the heated portion of the film selectively etch resistant as compared with the surrounding silicon material. The Laff et al technique, although potentially useful for upgrading large areas of silicon for solar cells, is not suitable for complex microelectronic structures where even localized heating to 1000.degree. C. can affect doping levels and interdiffusion of dopants into the device structure.
Thus, it would be highly desirable to find an inorganic resist material which does not swell upon development, is low in cost, and etches at a different rate upon exposure to low level heat than the unheated material.